Crystallization from solution of chemically active compounds or their intermediates is the typical method of purification used in industry, particularly the pharmaceutical industry. The integrity of the crystalline structure, or crystal habit, that is produced and the particle size of the end product are important considerations in the crystallization process.
For pharmaceuticals, high bioavailability and short dissolution time are desirable or often necessary attributes of the end product. However, the direct crystallization of small sized, high surface area particles is usually accomplished in a high supersaturation environment which often results in material of low purity, high friability, and decreased stability due to poor crystal structure formation. Because the bonding forces in organic crystal lattices generate a much higher frequency of amorphism than those found in highly ionic, inorganic solids, "oiling out" of supersaturated material is not uncommon, and such oils often solidify without structure.
Slow crystallization is a common technique used to increase product purity and produce a more stable crystal structure, but it is a process that decreases crystallizer productivity and produces large, low surface area particles that require subsequent high intensity milling. Currently, pharmaceutical compounds almost always require a post-crystallization milling step to increase particle surface area and thereby improve their bioavailability. However, high energy milling has drawbacks. For example, such milling may result in yield loss, noise and dusting, as well as unwanted personnel exposure to highly potent pharmaceutical compounds. Also, stresses generated on crystal surfaces during milling can adversely affect labile compounds. Overall, the three most desirable end-product goals of high surface area, high chemical purity, and high stability cannot be obtained using current crystallization technology.
One standard crystallization procedure involves contacting a supersaturated solution of the compound to be crystallized with an appropriate "anti-solvent" in a stirred vessel. Within the stirred vessel, the anti-solvent initiates primary nucleation which leads to crystal formation and crystal digestion during an aging step. Mixing within the vessel can be achieved with a variety of agitators (e.g., Rushton or pitched blade turbines, Intermig, etc.), and the process is done in a batchwise fashion.
When using current reverse addition technology for direct small particle crystallization, a concentration gradient can not be avoided during initial crystal formation because the introduction of feed solution to anti-solvent in the stirred vessel does not afford a thorough mixing of the two fluids prior to crystal formation. The existence of concentration gradients, and therefore a heterogeneous fluid environment at the point of initial crystal formation, impedes optimum crystal structure formation and increases impurity entrainment. If a slow crystallization technique is employed, more thorough mixing of the fluids can be attained prior to crystal formation which will improve crystal structure and purity, but the crystals produced will be large and milling will be necessary to meet pharmaceutical industry bioavailability requirements.
Another standard crystallization procedure employs temperature variation of a solution of the material to be crystallized in order to bring the solution to its supersaturation point, but this is a slow process that produces large crystals. Despite the elimination of a solvent gradient with this procedure, the resulting crystal characteristics of size, purity and stability are difficult to control and are inconsistent from batch to batch.
Impinging jets are routinely used for micromixing in reaction injection moulding (RIM) technology in the plastics industry, but not for the purpose of causing crystallization. The use of an impinging jet device in a crystallization process to achieve intense micromixing is novel. Whether feed material is relatively pure or impure, the use of impinging jets results in crystal characteristics superior to those that result from standard crystallization methods.